• Temperature Variations on Earth

    The entire planet Earth is the same distance from the Sun, about 93 millions miles, so why are all locations not the same temperature? There are several reasons for temperature variation.

    A diagram showing the tilt of the Earth's axis.
    The axis of the Earth currently tilts approximately 23.5 degrees from the perpendicular (dashed line) to its orbital plane.
     

    With a single click of your computer mouse, you can access web pages in Australia (like this Brisbane storm chaser's photo gallery). In this case, it's a small world indeed. With regard to seasons, however, Australia is a world away. While summer temperatures can reach the 90s in Australia during middle to late January, here in New York, we may be freezing with high temperatures only in the single digits or teens. To understand such seasonal disparity between hemispheres, recall that the axis of rotation of the Earth tilts at an angle of 23.5 degrees away from the line drawn perpendicular to the plane of our planet's orbit around the sun.

    The curvature of the Earth's surface along with the tilt of our axis both play a role in where and how much Earth is heated. Just so you're not in the dark with regard to this claim, direct your attention to this interactive tool. Please note that when the light from the Sun strikes the surface at a rather direct angle, the light focuses on a rather small area. In other words, the light is intense. On the other hand, if you interactively move the rays so that the light strikes the surface at a more oblique angle, the light spreads over a larger area (notice this is the case near the poles), making the light less intense. If you're not convinced, find a dark room and a flashlight and try it out for yourself.

    The sun's radiation, for all practical purposes, travels in parallel "rays" of energy because of its distance from the Earth. Therefore, the curvature and tilt of the Earth controls the heating power of insolation (the amount of solar radiation per unit area) and, on a grander scale, the seasons. You might wonder that, if the tilt of the Earth is 23.5 degrees, wouldn't the amount of insolation at any specific place on Earth be constant, resulting in a climate without seasons?

    Obviously, it's not. To explain why, I'll start with the observation that the Earth's axis points to the same position in space (namely, toward Polaris, the North Star). Thus, as the Earth travels in an elliptical orbit around the sun, the northern hemisphere can be tilted toward or away from the sun, depending on its orbital position.

     

    When the Northern Hemisphere tilts toward the sun, solar energy strikes the ground more directly during the daytime. Like a nearly downward pointing flashlight shining directly on the surface, concentrated sunlight has heating power consistent with the elevated temperatures of summer. Note that, at the start of summer, the Arctic Circle, which spans from about 66.5 degrees latitude to the North Pole, lies in total daylight. Thus, "nighttime" forecasts such as "partly sunny and cold" are not far-fetched at Barrow, Alaska, the northernmost office of the National Weather Service at approximately 71 degrees latitude.  Check out this time-lapse movie of the Arctic sun (note that while it dips towards the horizon, it never sets).

    At the same time the sun's rays are striking the ground more directly in the Northern Hemisphere, the Southern Hemisphere tilts away from the sun and sunlight strikes at an oblique angle (like light from a nearly horizontally held flashlight spreading out over a large area). This diffuse sunlight has low heating power that is consistent with the typically low temperatures of winter.

    Now click on "winter," "spring," and "fall" in the animation and investigate firsthand the effects of sun angle on seasonal heating power. Astronomically speaking, winter begins in the Northern Hemisphere on the winter solstice (on or around December 22) when local midday rays of the sun shine at right angles on the Tropic of Capricorn (approximately 23.5 degrees South latitude). Astronomical winter ends and spring begins on the spring equinox (on or around March 21) when local midday rays of the sun shine directly on the equator. Astronomical spring ends and summer begins on the summer solstice (on or around June 22), when local midday rays of the sun shine directly on the Tropic of Cancer (approximately 23.5 degrees North latitude). Astronomical summer ends and fall begins on the autumnal equinox (on or around September 23), when local midday rays of the sun shine directly on the equator.

    Top-ten 2-day snowfall totals for State College.
    Top-ten 2-day snowfall totals for State College, PA. (larger version) Notice that 5 out of the top 6 snowfalls occurred in meteorological spring (green shading) rather than winter (blue).
    Credit: David Babb

    Forecasters use different criteria to determine the "meteorological seasons." For example, meteorological winter runs from December 1 to February 28, a period that statistically includes the three coldest months of the year. Meteorological summer runs from June 1 to August 30, a period that includes the warmest three months of the year. Another definition from a different study has winter running from December 5 to March 5 and summer from June 5 to September 5, which, respectively mark the coldest and warmest 90-day periods of the year for many cities.

    I caution you, however, not to put too much stock in seasons -- either astronomical or meteorological -- when it comes predicting individual weather events. To see what I mean, consider the average monthly snowfall for State College, PA.  As expected, most of the snow in this location occurs during meteorological winter (December, January, February).  However, record snowstorms in State College tend to occur during the spring (not necessarily winter). Check out the graph to the left, showing the top-ten 2-day snowfall totals for State College. Five out of six of the largest snowstorms occurred in March, the greatest occurring in late March, 1942, when 30.5 inches crippled the region. Perhaps an even better example is Snowvember from here in Elma, Marilla, Cowlesville, West Seneca, Orchard Park and other Buffalo southtowns and eastern suburbs. That entire event actually occured in Meteorological and Astronomical Autumn (or fall). 

    You should realize that while sun angle is the driving factor for seasonal temperature variations, there are other factors at play as well.  For example, consider this NASA movie from 2000-2001, which shows the rhythms of the most intense ultraviolet radiation coincide with the most direct rays of the sun (around the summer solstices). Of course, there's nothing surprising about this movie. But what may be surprising to you is that average air temperatures lag behind the astronomical lead of the sun's most direct days.

    To see what I mean, let's return to Pittsburgh's plot of annual average high temperatures (below). Note that the maximum daily temperature occurs in the latter half of July, long after the summer solstice (the day when the midday rays of the sun strike Pittsburgh at their most direct angle). Similarly, the coldest days do not occur until the latter half of January. The bottom line here is that the greatest average daily high temperature at Pittsburgh does not occur on the day with the most intense radiation. In reality, the greatest average daily high occurs about a month later when the sun's rays strike Pittsburgh at less direct angles.

    The annual variation of average daily high temperatures at Pittsburgh, Pennsylvania.
    The annual variation of average daily high temperatures (dark curve) and daily extreme high temperatures (red represents record high maximums and blue represents record low maximums) at Pittsburgh, Pennsylvania.
    Credit: Earth System Research Laboratory

    To understand why this happens in the most simple terms, imagine that you take a piece of cold pizza from the refrigerator and place it in a preheated oven. After a minute or so, you get impatient and remove the pizza from the oven. In salivating expectation, you take your first bite and are immediately annoyed that it's still cold. You turn up the oven to maximum and cook the pizza another minute. But, alas, it's still not piping hot like you would prefer. Lesson learned: It takes time for cold pizza to heat up in an oven.

    And so it is with the Earth's atmosphere. Like cold pizza, the atmosphere, chilled by winter's refrigerator, takes time to warm up, not reaching its highest temperature, on average, until after maximum Sun intensity around the summer solstice. As we'll see a bit later, temperature lags often occur on the daily temperature cycle as well.  That is, the maximum solar insolation occurs around local noon but the high temperature for the day usually is recorded in the mid- to late-afternoon between 2 and 4 pm.

    Prioritize...

    In this section, make sure that you can describe the effects of latitude, altitude, and proximity to large bodies of water on large-scale, seasonal temperature trends.

    Read...

    In the last section you learned that seasons (that is, yearly temperature trends that occur over a large region of the Earth) are created by the tilt of the Earth and that the amount of solar radiation impinging on a surface depends on the angle at which it strikes the surface. If we look at slightly smaller regions over shorter time spans, we notice that the average surface air temperature across the Earth's surface is highly location dependent. The "big three" controllers of temperature based on location are latitude, altitude, and proximity to large bodies of water.  Let's examine each of these factors.

    Latitude

    You might have noticed that during the winter, the sun at local noon isn't as high in the sky as it is in the summer. The maximum height that the sun reaches at local noon on any given day (and thus the maximum angle at which the radiation strikes the Earth) depends on the latitude of your location. For example, consider two locations: Bismarck, ND (located at 46.5N latitude) and Oklahoma City, OK (located at 35.2N latitude). Around the time of the winter solstice (December 21st), the sun's highest point in the sky at Bismarck is only approximately 19.8 degrees off of the horizon.  At Oklahoma City however, the sun reaches a maximum angle of approximately 31.1 degrees off of the horizon. This means that the sun's radiation will strike the surface at angles of 19.8 and 31.1 degrees, respectively. If we perform the same calculations for a date located near the summer solstice (June 21st), we find that the radiation will be striking the earth at these locations at angles of 66.7 degrees (Bismarck), and 78.0 degrees (Oklahoma City). Note: if you want to calculate the sun angles for your own location, check out the NOAA solar calculator. So, what does this mean in terms of the amount of solar radiation received by each location? 

    The amount of radiation that strikes a unit area of the Earth's surface depends on the intensity of the radiation and the angle at which the radiation strikes the surface (called the "angle of incidence"). More specifically, we find that very little radiation per unit area strikes the surface at low angles of incidence (near 0 degrees), and that the maximum amount of radiation hits the surface per unit area if the surface is perpendicular to the incoming radiation (the angle of incidence is 90 degrees). Remember from the last section that when a beam of radiation strikes a surface at an angle, it is spread over a fixed area. The smaller the angle of incidence, the larger the area over which the radiation is distributed -- making it weaker. 

    I have already demonstrated that locations on the Earth experience different maximum sun angles based on their latitude. Therefore, on any given day, the clear-sky amount of solar energy available at a particular location is also a function of its latitude. For example, at Bismarck on December 21, the radiation is only about 34% of the maximum possible (often called the "direct beam"). On June 21, Bismarck is experiencing around 92% of the total sun's radiation per unit area. Oklahoma City on the other-hand, is experiencing a range of 52% to 98% of the sun's total radiation throughout the year. The net result is that Oklahoma City consistently receives more radiation than Bismark, ND throughout the year. Furthermore, the higher-latitude city (Bismarck) experiences a greater variation in radiation amounts than does Oklahoma City. With that in mind, check out the average temperature comparison below.

    The annual variation of average daily high temperatures at Oklahoma City, OK and Bismarck, ND.
    The annual variation of average daily high temperatures at Oklahoma City, OK (green curve) and Bismarck, ND (purple curve). The difference in latitude is the primary factor in the temperature difference between the two cities.
    Credit: Data supplied by the Earth System Research Laboratory

    As we would expect, Oklahoma City is, on average, warmer than Bismark, ND. In addition, we notice that Bismarck has a much wider range in average temperatures than does Oklahoma City (average high temperatures in Bismarck increase from about 20ºF in January to about 85ºF in July--a range of 65ºF, while Oklahoma City's average high temperatures increase from about 44ºF in January to 95ºF in July--a range of 51ºF). Lesson learned: All else being equal, the larger a location's latitude, the colder its average temperatures will be and the the more extreme the variation between summer and winter season temperatures.
     

    Altitude

    In the section on infrared satellite imagery, we established that temperature decreases with altitude. In fact, the average decrease in temperature with height is about 6.5 degrees C per kilometer (or 3.6 degrees F per 1000 ft). The reason why temperature decreases in the lower troposphere is that the primary heat source in this region is the Earth's surface. This is because the Earth's atmosphere is relatively transparent to solar radiation -- only the surface absorbs the lion's share of this radiation. The absorption of solar radiation, of course, warms the ground, which then transfers its heat to the atmosphere via a variety of processes. The end result is that the farther away from the Earth's surface you are, the colder the surrounding air. 

    Now you might be asking why mountainous regions tend to be colder than their low-land counterparts... after all, the mountain tops are still the surface of the Earth and thus should have the same heating properties as lower elevation surfaces. The key to this conundrum is that while the surface of the mountain top may indeed heat up just like any similar surface, the air surrounding the mountain top is vastly cooler than air at lower elevations. Therefore, as air near the surface of a mountain warms, it is quickly displaced by the much cooler air (we'll discuss why this happens later in the course). If the wind is blowing, this effect is compounded. Lesson learned: Surface temperatures are, in general, much cooler at higher elevations than at lower elevations.

    As an example, consider two cities: Columbia, Missouri and Eagle, Colorado.  Both cities lie at the same latitude and are located in the center of the United States. The difference between these two cities is elevation. Columbia's elevation is 705 feet above sea-level; Eagle's elevation is 6600 feet above sea-level.  Below is a comparison of the mean high temperatures for these two cities.

    The annual variation of mean maximum temperatures at Eagle, Colorado, and Columbia, Missouri.
    The annual variation of mean maximum temperatures at Eagle, Colorado (purple curve), and Columbia, Missouri (green curve). Note that Eagle's higher elevation of 6600 ft causes it to have a lower average high temperature compared to Columbia (located at 705 ft).
    Credit: Data supplied by the Earth System Research Laboratory

    As you can see in the graph, Eagle's higher elevation causes it to have a slightly cooler average high temperature than the lower-elevation city of Columbia.  

    Proximity to Bodies of Water

    To begin our discussion on the effect of large bodies of water on local temperatures, consider this color-coded temperature map (constructed from NASA satellite data). The top map indicates the average daytime air temperatures in January 1979, and the middle map represents the average nighttime temperatures during the same month (on both maps, brown represents the hottest regions and temperatures decrease from red to yellow to light blue to dark blue, which represents the coldest regions).

    The bottom map represents the difference between daytime and nighttime temperatures during January 1979. The whitish appearance of Earth's oceans means that there was little or no change between daytime and nighttime temperatures over the course of the month. What this map is telling you is that water is particularly slow to heat or cool -- much slower than land. In fact, it requires a great deal of energy to change the temperature of water even a fraction of a degree. Therefore, we might expect locations that are near large bodies of water to have average temperature curves that exhibit smaller ranges between the seasons. 

    If you examine the average temperatures for a west-coast city such as San Francisco, where prevailing winds blow off the ocean most of the time, you can observe the moderating influence of the Pacific which limits the variation in temperature from day to day and month to month. Indeed, note the relative flatness of the plot of daytime average high temperatures at San Francisco compared to St. Louis, Missouri (both cities lie at approximately the same latitude). The flatness in San Francisco's trace of daily average highs indicates a smaller annual variation in temperature. Indeed, average daily highs during summer at the "City by the Bay" are not nearly as high as St. Louis. During winter, however, the average daily highs in San Francisco are higher than St. Louis, again due to the moderating influence of the ocean. Practically speaking, the Pacific Ocean keeps San Francisco from getting hot in summer and cold in winter.

    The annual variation of mean maximum temperatures at San Francisco, California, and St Louis, Missouri.
    The annual variation of mean maximum temperatures at San Francisco, California, and St Louis, Missouri. Note that the annual variation at San Francisco is smaller than St. Louis, indicating the moderating effects of the Pacific Ocean.
    Credit: Data supplied by the Earth System Research Laboratory

    Oceans do not own a monopoly on moderating temperatures. To a lesser extent, large lakes, rivers and seas modify air temperatures. For example, the moderating effects of Lake Ontario and the Finger Lakes transform the western New York into a favorable place to grow grapes for making wine. Why? The reason that water can be such a modifier of temperatures stems from water's tendency to resist rapid temperature change. Water has what is called a high heat capacity, meaning that it requires a great deal of energy to raise the temperature of water even one degree. You've noticed this if you've ever jumped into a cold lake on a hot early June afternoon.  Brrrr!  We often say that the water hasn't "warmed up yet," but what we really are saying is that the water has not absorbed enough energy to raise its temperature to equal that of the ground (which has a more rapid temperature response to solar heating). 

    In the fall, the opposite is true of course. Getting back to our grape-growing question, because large bodies of water cool much more slowly than land, milder air overlying the Finger Lakes delays the first frost (courtesy: PlantMaps.com) of autumn, extending the growing season and allowing grapes to adequately ripen before harvest (note in the image the later average date for the first frost in the region south of Lake Ontario).  We say that bodies of water cause temperatures to "lag" those farther away from the water. This means that air temperatures surrounding large bodies of water will stay milder in the fall and winter, but will also be slow to warm during the spring and early summer.

    Now that we've examined large-scale, surface temperature controllers, let's zoom in on the surface of the Earth and really examine how radiation from the sun (and other sources) affects the temperature at a given location. 

     

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    Prioritize...

    By the end of this section, you should be able to list the three primary components of radiation that play a role in surface temperature trends. You should also be able to construct a simple energy budget given values for each of the three components and determine if temperature should increase or decrease.

    Read...

    An ink pen and ledger.
    The first step in figuring out the surface temperature trend is keeping track of the energy that is absorbed and emitted by the surface.
    Credit: Public Domain

    You learned early on in this course that temperature is a measure of the motion (or vibration) of molecules within a substance. The fact that the molecules are moving means that temperature must be a measure of energy -- often referred to as "thermal energy" (or "heat energy").  Therefore, understanding the temperature change at a particular location is simply a matter of identifying all of the ways that energy enters, leaves, changes form, or is transferred at that location.

    To begin this process, let's begin with identifying how energy enters or leaves a location via radiation. There are three main forms of radiation to consider: short-wave down (downwelling solar radiation), long-wave down (downwelling infrared radiation from clouds and the atmosphere), and long-wave up (upwelling infrared radiation from the Earth's surface).  The key point to remember is: if more radiation is coming in than leaving, the surface will heat up; if more radiation is leaving than coming in, then the surface will cool. You might be asking, "How do I know if there is a net loss or gain of radiation?" The answer... make a budget.

    Every financial planner will tell you that you need to make a budget so that you can see exactly what money is coming in and what you are spending. This is important information to have if you wanted to know if your bank account was growing or shrinking over time. Likewise, we can make a radiation budget in order to keep track of the radiation that is being absorbed and emitted by a surface. Unlike most household budgets, however, our radiation budget is fairly simple. We treat all downwelling radiation as income (that is, a positive contribution), and upwelling radiation as an expense. By examining the net accumulation or loss of radiation we can determine if there will be an increase or decrease in temperature. To get started, let's look at each of these three budget categories in more detail.

    Downwelling Solar Radiation

    You would hardly be surprised to learn that the sun plays a large role in supplying energy to the surface of the Earth (during the daylight hours, of course). Remember from a previous lesson that the sun's peak emission lies in the visible spectrum. This is convenient for us on Earth because the atmosphere is transparent to visible light. Therefore, most of the energy received at the Earth's surface is in the visible spectrum. So, how much radiation does the Earth's surface receive from the sun? Check out the plot below showing the 24-hour plot of irradiance for a location near State College on March 17, 2012. Irradiance is the power (energy per unit time) of the radiation striking a surface per unit area. In other words, at the sun's peak on this date, it's as if eight 100-watt light bulbs were shining on each square meter of the earth.

    A plot showing the downwelling solar radiation.
    A plot showing the downwelling solar radiation at a location near State College, PA on March 17, 2012.  Notice the sine-shaped curve that results from the angle dependence of the radiation per square meter.
    Credit: Earth System Research Laboratory

    The first thing that you should notice about the shape of the graph is that the sun only contributes radiation for a portion of the 24-hour day (I bet you knew that). Next, notice that the shape of the radiation curve looks like the top half of a wave. This is a direct result of the sine dependence of radiation per unit area on the angle of incidence that we discussed in the last section. I should also point out that the time of year can dramatically change this curve as well. For example, compare similar plots for solar irradiance on December 11, 2011 and June 2, 2011. As we've previously discussed, in the winter hemisphere the sun transcribes a lower arc in the sky, meaning that the duration of sunlight is less and the sun is lower in the sky at mid-day.

    Besides seasonal changes in sun angle, there is only one other factor that can affect the solar "income." To see what I mean, check out this solar irradiance plot for March 18, 2012. Why the jagged appearance in the plot? If you answered "clouds," then you're spot-on!  Clouds can block varying degrees of the downwelling solar radiation -- anywhere from partly cloudy as seen in the previous plot, to completely overcast (now that's a grey, dreary day!).

    So how do we enter this "income" on our energy budget balance sheet? Just like you don't get to keep all of the income you make, the surface isn't allowed to keep all of the radiation that strikes the surface, either. The amount of the sun's visible light that is absorbed by the surface and converted to heat energy depends on surface albedo. Remember that albedo is the ratio of reflected light compared to the total incoming amount. So an albedo of 30% (the Earth's average albedo) means that 30% of the incoming radiation will be reflected back to space while 70% will be absorbed. For dark surfaces (tilled ground, forests, asphalt) the surface albedo is much lower, allowing for more of the radiation to be absorbed by the surface. However for surfaces such as sand or snow cover, a great deal of the incoming solar radiation is simply scattered back to space. To account for the surface albedo, we might write the surface's solar income as: (1 - albedo) * downwelling solar.

    Next, let's look another source of income for the surface... Downwelling IR. 

    Downwelling Infrared Radiation

    Did you know that the amount of infrared radiation the Earth receives from the atmosphere is, on average, comparable to (if not greater than) the incoming solar radiation for a 24-hour period? Pretty amazing!  Keep in mind that, even though it’s way, way hotter, the sun occupies much less of the sky than our atmosphere. Moreover, the atmosphere radiates infrared radiation all day and all night. And like the persistent tortoise, slow and steady often wins the race in terms of radiation.

    To see what I mean, examine the radiation plot below. This is a similar plot to the one I have used above, only with downwelling IR added. Notice that the downwelling IR radiation is, on average, around 250 Watts per square meter and is somewhat constant throughout the day. To understand where this radiation comes from, remember that the atmosphere is a fairly efficient absorber of IR radiation due to atmospheric gasses such as water vapor and carbon dioxide. In turn, these gasses emit IR radiation as well as they absorb it, and thus, some of this emitted radiation makes it way back down to the surface. If I add up the total contribution to the downwelling IR radiation, I get a value of approximately 6000 Watt-hours per square meter. Likewise, if I add up the total solar contribution, the value comes out to be around 6100 W-hrs/m2. In fact, at this latitude, the atmosphere's IR contribution usually exceeds the solar contribution during the months of November through February. An unexpected result, I think.

    A plot showing the downwelling solar and downwelling IR radiation at the surface.
    A plot showing the downwelling solar (blue line) and downwelling IR (green line) radiation at a location near State College, PA on March 11, 2012.
    Credit: Earth System Research Laboratory

    What about clouds... do they also affect the downwelling IR radiation like they do the solar component? In fact, instead of limiting downwelling radiation (as they do for solar radiation), they actually increase downwelling IR radiation. Remember from the section on IR satellite images, clouds emit IR radiation in accordance to their temperature. The warmer they are, the more IR radiation they emit. In this light, think of clouds as "space heaters," emitting energy toward the ground. This is the reason that a cloudy night will tend to be much warmer than a clear night (all else being equal).

    Television weathercasters like Hale Stone, in an attempt to explain this common observation, will invariably explain higher temperatures on an overcast night by saying that "clouds act like a blanket." In fact, they do not!  Indeed, such comparisons are like ragweed in the garden of scientific truth and must be plucked, roots and all. To weed out the misconception that "clouds act like a blanket" at night, let's think about how blankets really work.

    Blankets work because they trap the air heated by your body, keeping it close to your body, and preventing it from mixing with colder air in the room. Think about it. Would your blankets still work if they were suspended above you like a tent with no sides?  Of course not!  Yet this is what we are asked to believe when we hear that "clouds act like a blanket." Clouds do not have the ability to trap warm air near the Earth's surface any more than a blanket tacked to your bedroom ceiling could help to keep you warm. They are simply a source of IR radiation that helps keep surface temperature warmer than they would be on a clear night.  Remember: Clouds are heaters, not blankets!

    Just to prove my point using actual data, check out this irradiance plot on March 12, 2012. Notice that the solar component contains the signature of increasing clouds. Likewise, note that the downwelling IR component increase from a clear-sky low of 250 Watts/m2 to nearly 400 Watts/m2. These must have been low, warm clouds indeed!  For another example, examine this plot from March 10, 2012.  It is indeed clear throughout the daylight hours, but can you tell whether the night before had clear skies?  Just look at the elevated values in the downwelling IR component from 0200Z to 1000Z. This tells you that clouds were present.

    Since downwelling IR is a second source of income for the surface, we need to add it to the solar radiation component. Let's designate the surface's IR income as: + downwelling IR. Now that we've accounted for the source of income, let's turn our attention to the surface's radiation spending habits.

    Upwelling Infrared Radiation

    Have you every heard someone say, "When the sun goes down, the Earth's surface begins to emit IR radiation to space...?" (A Hale Stone quote for sure!) Well, there is no need to ponder how the Earth knows when the sun sets. Remember from the section discussing the laws of radiation that everything that has a temperature emits radiation. This means that the ground is always emitting infrared radiation. This upwelling IR radiation is strongly temperature dependent. For example, on a chilly winter morning you could expect an upwelling IR value below 300 Watts per square meter, while on a hot summer's day you might see values exceeding 500 Watts per square meter. To see an example, consider the red curve on the plot below. Note that the IR emission increases dramatically during the day as surface temperature rises, and then drops more slowly over the nighttime period as the surface temperature slowly cools.

    A plot showing the downwelling solar, downwelling IR, and upwelling IR radiation at a surface location.
    A plot showing the downwelling solar (blue line), downwelling IR (green line), and upwelling IR (red line) radiation at a location near State College, PA on March 17, 2012.
    Credit: Earth System Research Laboratory

    We should remember when looking at this graph that the upwelling IR component represents radiation leaving the surface of the Earth and therefore should be subtracted from our energy budget (much like expenses are subtracted from income in a household budget). Let's designate the surface's expense term: - upwelling IR.

    The Complete Budget

    If you combine the three terms of the surface energy budget, you find that the net radiation absorbed by the surface is: (1 - albedo) * downwelling solar + downwelling IR - upwelling IR. This simple equation can be used to get a rough idea of the temperature trend of the surface. For example, using the graph above, look at the values of the three components at 1800Z.  Downwelling solar: 800 W/m2; downwelling IR: 300 W/m2; upwelling IR: 450 W/m2.  If we assume a surface albedo of 30% or 0.30, then our net radiation is: (1 - 0.3)*800 + 300 - 450 = +410 W/m2.  A positive value means that the surface will be increasing in temperature (because there is a net increase in thermal energy).

    Need another example? Take a look at the graph around 0600Z. Notice that the value of the downwelling solar is 0 W/m2 (it's night). The downwelling IR is around 300 W/m2 and the Upwelling IR is approximately 350 W/m2. Using our budget equation, we find that the net energy change is -50 W/m2. Since the surface is losing energy, then it must be cooling (as we would expect during a cloudless night).

    If you are interested in generating your own surface radiation graphs, you can do so at the SURFRAD Data Display page of the ESRL website. You can find many interesting cases to study by simply browsing the data for a particular location. As you look at the data, practice thinking about surface temperature trends based on the three radiation components.

    Quiz Yourself...

    Need some more practice thinking about surface temperature trends given a certain set of radiative parameters? Give the interactive calculator below a try...

     
    Test your knowledge of radiation budgets. First adjust the solar, IR-up, and IR down components and then try to figure out what the temperature response would be. Check your answer by moving your mouse over the "Check Surface Temperature Tendency" icon.
    Credit: David Babb

    To orient yourself, the surface temperature slider on the left controls the upwelling IR component, while the sun position slider controls the downwelling solar component. By hovering your mouse over the "Display surface Temperature Tendency" label, you can see a qualitative graph of the surface temperature trend. If you want to complicate the problem, check the "Cloud Present" check box. For the cloud, you are allowed to change its height (high/cold versus low/warm) and its thickness (which impacts the downwelling solar). You will note that with the cloud added, you are able to vary the downwelling IR component as well. After you select a scenario, try to figure out whether the temperature trend will be an increase or decrease, and whether it will be large or small. You might also compare similar scenarios -- for example, look at the nighttime temperature trend with a low cloud versus no cloud.

     

    Prioritize...

    After reading this section, you should be able to describe how thermal energy is transferred by conduction and convection.  Specifically, you should be able to discuss how each process allows thermal energy to move from the Earth's surface to the overlying air. Focus on the speed, the process, and the result of each mechanism.

    Read...

    There are three processes that transfer energy between the atmosphere and the Earth's underlying surface... conduction, convection and radiation. We've already talked about how the Earth's surface heats/cools in response to absorption and emission of radiation at the surface. We have also seen that the atmosphere's absorption and emission of infrared radiation is crucial to keeping the ground much warmer than it otherwise would be (and, therefore, the air in contact with the ground).

    However, radiation processes are not sufficient to explaining the temperature of the air. What we need to discuss now is how the heat from the Earth's surface is transferred to the atmosphere. For starters, I want to conduct a discussion on conduction, a "touchy subject" as you will shortly see.

    Conduction

    Recall from our definition of temperature that the molecules and atoms in warm objects have high kinetic energy, on average. The kinetic energy of molecules and atoms in cold objects is much more low key. When warm and cold objects come into contact, fast-moving atoms and molecules collide with slower ones, imparting kinetic energy as a result of the collision. To see what I mean, think of wildly dancing teenagers bumping into retired couples who are dancing cheek-to-cheek to a slow ballad. Collisions are unavoidable, and slow dancers gain unwanted kinetic energy and lurch awkwardly across the dance floor. Meanwhile, frenetic teenagers lose kinetic energy during collisions. For another visual example, check out this flash animation illustrating conduction. Click, hold and drag the right end of the metal bar into the furnace. As this end of the metal bar warms, heat energy will be conducted toward the left as fast-vibrating molecules collide with molecules vibrating more slowly.

    In light of this dance-floor metaphor, let's see what happens when a relatively warm object comes into contact with a cooler object. Not surprisingly, the warmer object gets colder as its wildly dancing molecules lose kinetic energy when they collide with the slower dancing molecules of the colder object. In turn, the colder object gets warmer as it gains kinetic energy during contact.

    For example, your hand feels cold when you grasp a metal object in your apartment or house. That's because metal has a high "thermal conductivity" (a measure of a material's ability to conduct heat energy). In other words, metal conducts kinetic energy rapidly away from the fast-vibrating molecules in your skin. As a result, your hand feels cool.

    Unlike most metals, air has low thermal conductivity. That's why porous materials such as wool (porous means that there are small pockets for air to occupy) are effective thermal insulators. So it shouldn't come as a surprise that conduction between the ground and the overlying air proceeds at a relatively slow pace. To get a sense of what I mean, suppose you were to press a slab of wood a few inches thick against a hot burner on your kitchen stove that is much too hot to touch. The temperature of the wood in contact with the oven burner is pretty darn close to that of the burner. But the top of the wood slab can be touched without any pain. Why? Answer: The block of wood takes time to heat up. Indeed, the thermal conductivity of wood is sufficiently low so that the transfer of molecular kinetic energy through the thickness of the wood is slow.

    The low thermal conductivity of the wood slab on your kitchen stove is akin to a slab of air overlying the hot ground on a sunny summer day. After sunrise, the ground typically warms rapidly as it absorbs relatively intense solar radiation. Unlike the transparent upper layers of the ocean, incoming solar energy concentrates in the first few inches of the ground (even on a sunny, hot day, you don't have to dig very far to reach cool soil). In turn, a very thin layer of air in contact with the ground warms dramatically, albeit rather slowly.

    On paved roads, temperatures in this thin layer of air can reach as high as 140 degrees. Air temperatures at nose-level, however, are, say 85 degrees, marking a rapid drop-off with height. Hot bare feet (ouch!) but tolerable nose-level temperatures prevail, in part, because of the air's low conductivity. However, there's something else that helps to keep noses relatively comfortable while bare feet sauté on a simmering road during summer.

    Fried egg on an asphalt surface.
    So hot you can fry an egg on the sidewalk. Have you every heard that saying? It turns out that although the Earth's surface (especially solar radiation-absorbing black surfaces) can indeed reach temperatures exceeding 130F, it's still not quite hot enough to fry an egg. By the way, the photographer admitted that she "helped" the egg out a bit.

    Convection

    Convection, the locomotion mode of heat transfer, limits the thickening of the layer of very hot air in contact with the ground on a sunny, hot day. Just like a hot-air balloon lifting off the ground, blobs or "parcels" of hot air rise from the ground, carrying hot air skyward. This transfer of heat energy away from the ground by the vertical movement of air is called "free convection" or "natural convection" (in general, convection can occur in any fluid in any direction).

    A hot air balloon.
    A hot air balloon rises because warmer air is less dense than cool air. Since the balloon is less dense than the air around it, it becomes positively buoyant. How do hot air balloon get their warm air you ask? ... with large burners located above the basket.
    Credit: Hot Air Balloons / fdecomite / CC BY 2.0 (main image) and Flames / Eric BC Lim / CC BY 2.0/ (burner).

    To understand the nuts and bolts of natural convection, we start with the concept of buoyancy. Suppose that, while taking a swim, you submerge your favorite beach ball and then let it go. In a heartbeat, the beach ball will bob to the surface of the water. In scientific terms, the beach ball is positively buoyant. Now submerge a rock and then release it. It falls to the bottom of the pool because the rock lacks sufficient positive buoyancy to keep it afloat. Formally, we say that the rock has negative buoyancy.

    What makes the difference in the buoyancy between a rock and a beach ball? The answer is density. Formally, the density of an object is its mass divided by its volume. The beach ball has a relatively large volume and small mass, making its density rather small and far less than the density of water, which is approximately 1000 kilograms per cubic meter (1000 kilograms equates to 1.1 tons, so a cubic meter of water carries a lot of weight). A rock made of granite has a density of about 2500 kilograms per cubic meter (2500 kilograms converts to over three tons, so, if you thought a cubic meter of water was heavy, a cubic meter of granite is even heavier).

    From our swimming pool experiments, we arrive at the following generalization: an object immersed in a fluid (water, air, etc.) is positively buoyant if the density of the object is less than the density of the fluid. Moreover, the magnitude of the buoyancy force depends on the difference in densities between the submersed object and the fluid - the greater the difference, the greater the buoyancy force.

    Okay, let's take our discussion out of the water and into the air. For the time being, let's start with a "parcel" of air ("parcel" is just a fancy name for a generic blob of air that we assume, for sake of argument, does not interact with surrounding air). There are several factors that can cause the density of the air to change. In this section, we will restrict our focus to the effects of temperature on air density. For the record, typical values for the density of air at sea level range from approximately 1.2 to 1.3 kilograms per cubic meter (about 1/800 of the density of water). The operative word is "typical" because temperature changes result in density changes, which, as you might suspect, have implications on the buoyancy of the air.

    To understand the implications of changes in temperature to changes in density, let's conduct a simple experiment. First, I'll place a soda bottle in a pan of cold water for a few minutes and then cover the opening with a cheap party balloon. With the balloon sealing off the air in the bottle, we've isolated a "parcel" of air whose mass will remain constant provided we don't remove the balloon. Now, I'll heat the container of water in which the bottle sits. As the water heats the bottle and the inside air, air molecules increase their kinetic energy, and the air inside the bottle expands and inflates the balloon. In other words, the volume occupied by the "air parcel" is now larger, despite the mass of the air remaining the same. It follows that the air density, which is mass divided by volume, is now less.

    A bottle covered by a balloon illustrating how gasses expand when heated.
    An empty soda bottle with a balloon over its mouth was initially immersed in a pan of cold water (left). Then the pan was heated, causing air inside the bottle to expand as it warmed (right).
    Credit: David Babb

    We arrive at the following important result: Increasing the temperature of the air inside a parcel causes its density to lower (and vice versa). In turn, the positive buoyancy of the parcel increases and, as a result, it shows a tendency to rise if it's "submersed" in air with higher density.

    Okay, I'm now ready to connect this discussion on density and buoyancy to natural convection. On a summer day, the sun heats the ground and, in turn, the ground heats a thin layer of air in contact with it. But the ground heats the overlying air unevenly, so there are spots that are hotter than others. For example, think about a sunny summer day and the torridly hot air in contact with the concrete surface of a parking lot. Now think about the cooler air that overlies the surrounding grassy area. The air over the parking lot is less dense than the surrounding air and therefore more positively buoyant. In turn, air parcels rise more readily from the parking lot, transferring heat energy upward. This transfer of heat energy is, of course, a consequence of natural convection. Manifestations of natural convection vary from the sensational cumulonimbus that reach miles into the sky to the more subtle "thermals" routinely ridden by hawks, glider pilots, and hang gliders.

    To the naked eye, these thermals, which are currents of rising air associated with natural convection, are often invisible. But a striking form of imaging called Schlieren photography (explanation) allows for us to visualize the rather subtle variations of density of parcels of air rising from a relatively warm object like the ground. Below is a Schlieren image of Lee Grenci. Notice the thermals of warm buoyant air that can be seen rising from his skin.

    A Schlieren photograph of author, Lee Grenci.
    A Schlieren photograph of original course author, Lee Grenci. Schlieren photography allows very small density differences in a fluid (usually the air) to be seen. In this image you can see thermals of warm air rising off of Mr. Grenci's skin. If you are interested check out this video showing more of Lee's "heat".
    Credit: David Babb

    What should you take away from this discussion? Probably the most salient message here is that air temperature generally decreases with increasing height above the Earth's surface. I caution that this general decrease in temperature only extends so high ... a concept we will explore in a future lesson. In the next section, I will introduce another one of the ground's helpers in controlling surface air temperatures -- the wind. Read on.

     

    Prioritize...

    After reading this section, you should be able to describe the term "latent heat" and be able to discuss ways in which condensation and evaporation affect local temperatures.

    Read...

    Dew Points and Nighttime Low Temperatures

    On sultry, very humid nights during the summer, lows in the 70s are common. However, in arid, desert regions, nighttime temperatures can fall considerably. For example, during the Gulf War in 1991-92, the military endured searing desert heat by day. Away from the tropical Persian Gulf, dew points were mercifully low, setting the stage for hot days to be followed by nights so chilly that soldiers needed blankets to stay warm. For further proof, check out the Sahara Desert in northern Africa in the global average temperatures for January, 1979. Note that the Sahara is hot during the day (top map) and cool at night (middle map), making this desert and others the areas of the world with the greatest diurnal change (lower map).

    Why do high dew points promote warm nights and why do low dew points favor cool nights? To answer these questions without adding complications, we first assume the wind is calm and the sky is clear.

    On a night with high dew points, ample water vapor emits infrared radiation to the readily absorbing ground, helping to retard its cooling rate. In turn, the ground, radiating at an elevated temperature courtesy of water vapor emissions, now provides a boost in infrared energy for water vapor to absorb and warm up. This radiative synergy between the ground and water vapor keeps the ground and the overlying air warmer at night, resulting in elevated low temperatures.

    On nights with relatively low dew points, however, reduced concentrations of water vapor limit the amount of infrared radiation that the ground receives from the atmosphere.  Like you expect when reducing a source of income, the ground's energy budget will run a much larger radiation deficit than when dew points are high. The result is that temperatures can fall like a rock...toward the dew point. Indeed, on clear, relatively calm nights, the dew point serves as a reasonable lower bound for the nighttime minimum temperature (apprentice forecasters should remember this crusty old forecasting tool).

    To see what I mean, check out the meteogram at Modesto, California, from 18Z on January 28 to 19Z on January 29, 2010.  After readings peaked at 57 degrees at 00Z on the 29th (4 P.M. PST on the 28th), temperatures started to gradually decline while the dew point stayed relatively steady in the low to mid 40s. Fog started to form at 05Z on the 29th as the temperature fell toward the dew point. The fog thickened with time as the temperature met the dew point a little before 12Z (4 A.M. PST) before dawn on the 29th. Note, on the station models displayed above the middle graph on the meteogram, that the sky was obscured by fog at 11Z and 12Z. Meanwhile, horizontal visibility approached zero at these times (read the horizontal visibility off the vertical axis on the right of the middle graph). Heavy fog, indeed.

    A meteogram illustrating a typical scenario when fog forms on a clear night with light winds.
    The meteogram at Modesto, California, from 18Z on January 28 to 19Z on January 29, 2010, illustrates a typical scenario when fog forms on a clear night with light winds.
    Credit: University of Wyoming

    When the temperature reaches and falls just a tad below the dew point (the limited accuracy of thermometers cannot adequately measure this very slight difference), net condensation occurs, and assuming a light wind to help to spread the chill near the ground through a thicker layer, fog forms. At this point, the temperature stabilizes. Until the fog dissipates, the temperature is married to the slowly changing dew point.

    The reason that the temperature does not, for all practical purposes, ever fall measurably below the dew point after fog forms is because heat energy is released during the process of net condensation (this heat energy is formally called latent heat of condensation -- "latent" means "hidden"). Though this new topic may seem like a bit of a digression from our discussion of controllers of temperature, rest assured that it's not. The release of latent heat of condensation when a fog forms at night is indeed another kind of a controller of temperature.

    To understand this "bonus" energy that's released when water vapor condenses into water, I feel obliged to showcase the entire "energy staircase" for ice, water and water vapor so that you can put the three phases of water into better context.

    An energy staircase showing the phase changes of water
    The energy levels associated with ice, water and water vapor can be thought of as a set of steps. Changing from one phase (solid, liquid or gas) to another requires either an addition of energy (stepping up) or a release of energy (stepping down).
    Credit: David Babb

    Referring to the image of the energy staircase, please note that the lowest level of energy corresponds to ice, the solid phase of water characterized by rigid bonds between relatively sluggish molecules. To reach the next step requires more heat energy to melt the ice. It takes approximately 80 calories of heat to melt one gram of ice.

    Once the rigid molecular bonds of the icy lattice are broken and ice melts into water, molecules jump to a higher energy step (state). Water molecules are very social, bonding easily with their neighbors. A few highly energetic, free-spirited water molecules can eventually break these bonds over time and escape to the vapor phase. Over shorter times, heat energy must be added to break all the bonds to allow all the water to rather quickly evaporate and enter the gaseous phase of water vapor (the highest energy step). For reference, the heat energy required for evaporation approximately equals 600 calories per gram of water.

    When water vapor condenses back into water, there's a step down in energy levels. But the energy used to evaporate water in the first place is never lost (a consequence of the conservation of energy). As water vapor condenses into water, latent heat of condensation, amounting to the original investment of 600 calories per gram, is released to keep the energy books balanced.

    Latent heat of condensation serves to arrest the general decline of temperatures after fog forms on a relatively calm and previously clear night. This observation supports the message sent by the meteogram at Modesto, California -- to within the accuracy of thermometers, the temperature does not fall below the dew point and the dew point can indeed be used as a reasonable lower bound for the minimum temperature on a generally clear and relatively calm night.

    As an apprentice forecaster, you must be able to weigh each of the controllers of nighttime temperatures for any city and town. Such forecasting prowess takes experience and perseverance. Later in the course, we will discuss strategies and guidance for making accurate nighttime forecasts.

    You might be asking what about evaporation? Can the latent heat of evaporation play a similar role in regulating temperatures?  Read on.

    Evaporating Precipitation and Daytime Temperatures

    When rain, snow or any other type of precipitation falls into the layer of air overlying the ground, some of the precipitation usually evaporates -- especially if there is a large spread between the temperature and the dew point below cloud base.  As you recall from the "energy staircase," evaporation of water requires energy. In the case of falling raindrops, the energy required to evaporate some of them (usually the smallest ones) comes from the air. The extraction of energy from the air results in the air temperature decreasing. Formally, "evaporative cooling" is the term used to describe the process by which the air cools as raindrops evaporate. Note: I often hear students say that the "rain cooled the air". This is not correct. Rain falling through the air has very little cooling effect. Rather, it's the evaporation of rain drops that actually cools the air.

    For winter buffs, snowflakes can sublime directly from solid to water vapor, extracting even more energy from the air and potentially lowering air temperatures dramatically. In fact, you will often observe that temperatures are above freezing despite snow being forecast. Then, low-and-behold, when it begins to snow, the temperature drops below freezing. This is because sublimational cooling of snowflakes lowered the temperature to below freezing. I often look at the dew point as an indication of how dry the air is, and thus how much evaporational or sublimational cooling I can expect.  If my dew points are in the lower 20s F before the precipitation starts (say the temperature is 35 F), I know that there will be some substantial sublimation and evaporation at the onset of precipitation.

    The bottom line is that when clouds precipitate, surface temperatures typically decrease via evaporational cooling, as the meteogram for Kuching, Malaysia, on February 1-2, 2010, clearly shows (below). Also note how the dew point increases near 06Z as evaporating raindrops falling from thunderstorms increase the amount of water vapor in the air.

    A meteogram showing the effects on temperature from a passing thunderstorm.
    A meteorogram for Kuching, Malaysia, from 06Z on February 1 to 07Z on February 2, 2010. Note how the temperature plummeted from 86 degrees at 03Z (11 A.M., local time) to 75 degrees at 05Z (1 P.M., local time) with the approach and arrival of a thunderstorm at 05Z.
    Credit: University of Wyoming

    It also turns out the evaporational cooling is not limited to falling precipitation. In fact, wet ground depletes the heating power of solar radiation because some energy goes into evaporating water instead of directly heating the ground. To show you what I mean, I will revisit the summer and early fall of 2000 in northern Texas and Oklahoma, one of the region's driest on record. At the Dallas-Fort Worth airport, for example, not a single drop of rain fell for 84 consecutive days, crushing the all-time record.

    With the landscape parched over northern Texas and Oklahoma, a large portion of the sun's energy went directly into heating the ground, keeping daytime high temperatures in the 90s and 100s. Then, on September 23-24, welcome rains came as showers and thunderstorms dumped up to four inches. The bad news was that the coverage of the rainfall was rather limited.

    Here is the radar's estimate of rainfall that fell on September 23-24. By way of review, recall that once it starts to rain, the radar's computer estimates the rate at which rain falls (rain intensity) over relatively short periods of time and then calculates a rainfall for that period. Next, the computer adds all such rainfalls to get a total rainfall from the time precipitation first began. You can see that most of the rain fell in the southeastern half of Oklahoma and parts of northwest Texas, leaving a large part of the drought-stricken region high and dry.

    On the first sunny day following the welcome rains (September 26), daytime air temperatures over the wet ground averaged a few degrees less than the surrounding dry landscape. This enhanced infrared satellite image shows the lower radiating temperatures of wet ground compared those corresponding to the hotter dry land (in red). Note the shape of the cooler ground fits almost perfectly with the shape outlined by the radar-derived rainfall (move your mouse over the image). Reiterating my point, lower air temperatures over the swath of wet land were a direct result of some of the sun's energy being spent on evaporation rather than heating the ground.

    A map showing Ground IR on the southern plains of the United States
    An enhanced-IR image of the southern Plains on the afternoon of September 26, 2000. Red shading indicates high radiating temperatures associated with hot, dry ground. Move your mouse over the image to superimpose the rainfall pattern. Note the lower surface temperatures where rain fell two days earlier.
    Credit: NOAA

    Dew points over the wet ground were also a few degrees higher, another clue that evaporation meant business. Incidentally, nighttime temperatures were slightly elevated over this region, indicative of the radiative effect of increased water vapor over wet ground (winds were light, so they did not advect moisture out of the area).

     

    Prioritize...

    After you complete this section, you should be able to analyze real situations involving temperature tendency. To do so, you should be able to correctly identify and apply basic temperature tools such as a radiation budget, advection, and latent heat effects to complex problems. Specifically, you should be able to explain the effects of snow cover, urbanization, and wind trajectories on temperature tendency.

    Read...

    In this lesson you have learned about several tools to help you asses the temperature tendency -- namely, large scale factors, making a radiation budget, heat transfer by conduction/convection, advection, and latent heat effects. Often, all of these factors must be considered when assessing the observed temperature pattern or trend. In this section, I present three classic cases where several factors affect the surface air temperature.

    Snow Cover

    During the day, fresh snow reflects some of the sun's energy back to space (meaning, less absorbed in-coming solar affects the radiation budget). Plus, a portion of the sun's energy absorbed by the snowpack goes toward melting the snow pack, as well as evaporating meltwater (meaning, the energy used in the phase changes is not available to warm the surface). As a result of these two factors, daytime air temperatures are sometimes noticeably lower than if there had been no snow cover. In the case shown below, chilly air overlying a snow pack retarded the advance of the leading edge of much warmer air from the south and west. Notice the dramatic temperature change from the western Dakota's to Minnesota.

    Images described adequately in text and image caption
    Snow covered the ground over the upper Middle West on February 14, 1995 (right panel), limiting maximum temperatures (left panel). Snow depth is expressed in inches and "T" means "a trace of snow".
    Credit: NOAA

    I point out that the effect of lower daytime temperatures in regions that have snow cover is much more pronounced over the Middle West, where there is a lack of forests. In the Northeast, for example, dense coniferous and deciduous forests offset the high albedo of snow, presenting a much darker appearance to space. In this visible satellite image, the Adirondack region of northern New York, though snow-covered, appears dark, primarily because of the thick forests that populate this beautiful region. Here, the underlying, highly reflective snowpack is masked by the dominating lower albedo of forests because trees lose the snow that accumulates on their limbs fairly quickly in the wake of a storm.

    A visible satellite image of the northeastern U.S. with a portion of New York State cirlced. The area inside the circle is darker than the surounding areas.
    In forested regions like the Adirondacks (circled), dark deciduous and coniferous trees offset the high albedo of freshly fallen snow.
    Credit: NOAA

    The effect of snowpack on nighttime temperatures can be also be dramatic. Snow readily absorbs and emits in a thin infrared portion of the electromagnetic spectrum. Thus, at night, snow emits infrared radiation with similar efficiency as bare ground. So why would there be any difference in nighttime temperatures over snow cover compared to bare ground?

    Good question. Generally speaking, a snowpack of two inches or more generally seals off heat energy stored in the ground (snow is an effective insulator, which is why people stranded in a mountain wilderness during winter will dig a protective burrow in the snow to stay warm). With heat energy stored in the ground effectively sealed off, the temperature of the snow surface drops precipitously on a clear, calm night. In turn, overlying air temperatures plummet, setting the stage for very low predawn-temperatures.

    By the way, air temperatures over snow cover can continue to fall for an hour or so after sunrise during winter because the low angle of the sun (and the pathetically low heating power) cannot stem the tide of infrared losses at the surface of the snow.

    So, is snow cover a new consideration that we have to add to our thinking, or is it just an application of one of the tools that we have already learned?  I hope that you have realized that snow cover simply changes the surface radiation budget has latent heat implications. If you know what these changes are (higher surface albedo, less conduction, etc.) you should be able to figure out the resulting effect on temperatures.

    Urban Heat Island Effect

    In the visible image of the upper Middle West in March, 2001 (on the left below), Minneapolis clearly stands out from the surrounding snow-covered rural areas. The higher urban temperatures have essentially melted most of the snow in the city. The dark splotch over Minneapolis in the twin infrared image (on the right below) reveals the higher radiating temperature of the city, offering positive proof of the warming effects of the urban heat island.

    Images described adequately in text and image caption
    The visible satellite image (left) shows that there is less snow over Minneapolis, MN (red circle) than surrounding rural areas on March 27, 2001. The lack of snow within city limits is caused, in part, by the effects of the urban-heat island. The corresponding infrared image on the right clearly indicates the higher radiating temperatures of the big city.
    Credit: NASA/GSFC

    On a sunny day with light winds, temperatures in any big city can be several degrees higher than surrounding rural areas as concrete and buildings voraciously absorb solar radiation due to lower albedos of many urban surfaces. Moreover, heat from cars, industry and other human activities accent the warmer city environment.  Meanwhile, over the surrounding countryside, higher albedos typical of vegetation along with water vapor released from trees and plants (which "sweat" in a process called "transpiration") serve to help to keep the daytime rural environment cooler than its urban counterpart.

    For another example of an urban heat-island, consider the southern city of Atlanta, Georgia. You can see the growth of "Hot-lanta" in this movie, which shows the evolution of dense urban environment (indicated in red) from the early 1970s to the late 1990s. On sunny days with light winds during the warm season, temperatures in downtown Atlanta can exceed readings over rural areas by as much as ten degrees.

    Satellite maps of Atlanta, Ga. Adequately described in the text and image caption.
    The urban and highly populated suburban area of Atlanta, shown on the left in dark and light gray (respectively), correspond to higher daytime temperatures (indicated in red on the right).
    Credit: Earth Observatory / NASA

    I should point out that, as winds increase, the temperature contrasts between urban and rural areas tend to diminish. That's because the wind stirs, blends and mixes the lower troposphere, thereby making surface temperatures more homogeneous. Widespread precipitation and its associated evaporative cooling also tend to equalize daytime temperatures between urban and rural environments (in much the same way that eddies help to diminish the effects of the nocturnal inversion).

    Though noticeable differences between urban and rural temperatures exist during the day, the effect of the urban-heat island stands out more dramatically at night, particularly on clear, calm winter nights that follow sunny days. This is because sunshine warms the city more than surrounding rural areas, so city temperatures are already higher than rural temperatures as the sun starts to set. The temperature gap between the warmer city and the cooler countryside widens throughout  the night as concrete and buildings, which absorbed plenty of solar radiation by day, slowly and steadily conduct, convect and radiate stored energy to the urban environment.

    Left image (urban) is almost completely covered with orange and yellow, while right image (rural) shows the presence of areas of lower temperature.
    These two false-color infrared images show the difference in nighttime radiating temperatures between urban (left) and suburban (right) areas. Yellows and oranges depict higher temperatures.
    Credit: Earth Observatory / NASA

    During summer, when dew points are high, urban nights can be oppressive as water vapor absorbs large amounts of infrared radiation emitted by buildings and concrete. This absorption causes the water vapor to warm and radiate at higher temperatures, thereby slowing or essentially halting nocturnal cooling in the city. As an example of Atlanta's urban-heat island, the two false-color infrared images (above) show the difference in nighttime radiating temperatures between urban (left) and suburban (right) areas.

    Now, I ask you the same question as before... is the urban heat island effect a new consideration, or an application of some of the the basic tools that we have already covered?  You should have noticed my use of radiative terms such as "albedo" in my description above. The urban heat island effect is just another modification of the surface radiative properties (with perhaps a little bit of latent heat effects as well). We also discussed the effects of mechanical eddies in temperature maintenance in an urban environment. Remember, the key in analyzing a situation is to see how each of the tools that you have learned about in this lesson come into play. Ask yourself: What does the radiation budget look like? Is there anything that is affecting heat transport from the ground to the air? Is advection a key player? What about latent heat affects? The answers to these questions will guide you in determining how the temperature will behave over any given period.

    Air Trajectories

    Surface map surrounding Lake Ontario. Stations at Toronto, Ontario and Watertown, NY are pinpointed.
    Note the land trajectory of the surface wind at Watertown, New York (in green) and compare it to the cooler lake trajectory at Toronto, Ontario (in red). As a result, temperatures differed by nine degrees at this time.
    Credit:

    As a final example, look at the surface map for April 7 (right) and focus on two stations -- Toronto, Ontario (colored red) and Watertown, New York (colored green). Note that the wind direction at both stations is east-southeast. Further note that the temperature at Watertown is 9 degrees higher than the temperature at Toronto, even though Watertown is farther north.

    The explanation for the disparity in temperature is a matter of trajectory. In the case of Watertown, the trajectory of the air arriving at the small lakeside town is over land. But, at Toronto, the trajectory of the air entering the large lakeside city is over water. 

    By now you should be thinking that this applied problem deals with heat transfer and advection (did you figure it out?). Remember that, in early April, Lake Ontario is still very chilly (check out this IR image from April 9, 2012). Air flowing over the lake begins to take on the temperature characteristics of the colder water. As the air crosses the shoreline, the temperature at Toronto behaves as if there were cold air advection taking place. The speed of the wind, the land/water temperature contrast, and the angle at which the air crosses the shoreline all contribute to the resulting temperature change for a location downwind of the lake. The lesson learned here is that you, as an apprentice forecaster, must always be aware of the trajectory of the air and the potential impact of that trajectory on local temperature advection. Does the air pass over a snowpack before reaching your town? Hot desert sand? A cold body of water?

    You may not have to consider air trajectories all the time (bodies of water warm up and snowpacks eventually melt, for example). But when you do, you'll need to be able to forecast the direction of the wind. We'll take up that issue in the next lesson.